In a liquid crystal display panel disclosed in U.S. Pat. No. 4,840,460 each pixel electrode, which faces a common electrode across a liquid crystal interposed therebetween, is divided into a plurality of subpixel electrodes of the same area, and control capacitor electrodes are provided each of which has a different area and faces one of the divided subpixel electrodes across an insulating layer sandwiched therebetween. The control capacitor electrodes in each pixel are interconnected electrically. Each control capacitor electrode and the subpixel electrode disposed opposite it across the insulating layer constitute a control capacitor, and the subpixel electrode and the common electrode facing it across the liquid crystal constitute a liquid crystal capacitor. These two capacitors are connected in series to each other. A drive voltage which is applied to a control electrode is divided by the two capacitors. The liquid crystal capacitors have the same capacity but all the control capacitors have different capacities. On this account, even if a voltage is applied to all the control capacitor electrodes in common to them, the liquid crystal capacitors are each supplied with a different capacitance-divided voltage. The threshold voltage of the liquid crystal (i.e. a voltage which is applied to the liquid crystal when the transmission of light through the liquid crystal display panel begins) is substantially constant all over the display panel. Hence, by controlling the voltage which is applied to the control capacitor electrodes, it is possible to control the numbers of subpixel electrodes from which capacitance-divided voltages higher and lower than the threshold voltage, respectively, are applied to the liquid crystal, and consequently, the divided regions of the pixel can be driven stepwise.
In the case of applying the liquid crystal display panel to a picture display including a gray level as in television, the drive voltage that is applied to each pixel of the liquid crystal display may take various values or magnitudes within a certain voltage range in accordance with the picture signal level. In a liquid crystal display panel of the type wherein each pixel is not divided into subpixels, as the drive voltage increases, a gray-level display is produced through utilization of a sloping range of a light transmittance curve from the rise to saturation of the transmittance of each pixel region. In the slope region of the light transmittance curve liquid crystal molecules are oriented obliquely to the substrate. Since the visual angle dependence of the light transmittance is large in this case, a proper angle of field for such a liquid crystal display panel is generally very small.
In the conventional liquid crystal display panel wherein each pixel is composed of a plurality of subpixels and the subpixels are supplied with voltages which differ in a sequential order as described in the aforementioned U.S. patent, however, the transmission of light through one subpixel region reaches saturation through the above-mentioned sloping range of the transmittance curve after starting as the drive voltage is increased, and then the transmission of light through another subpixel region similarly reaches saturation through the sloping range. In this way, the respective subpixel regions reach the saturation range through the sloping range of the light transmittance curve one after another. Hence, in the state of providing a display at an arbitrary gray level, the liquid crystal molecules are oriented obliquely to the substrate in one subpixel region at most but in the other subpixel regions the liquid crystal molecules are oriented vertically or horizontally to the substrate. By decreasing the number of subpixel regions in which the liquid crystal molecules are oriented obliquely to the substrate in the gray-level display as mentioned above, the area of a large visual angle dependence in the pixel region is reduced, and consequently, the average visual angle dependence of the entire pixel region can be reduced.
With the pixel structure disclosed in the U.S. patent, however, the subpixel electrodes are each separated by a gap or region to which no voltage can be applied, i.e. a region which does not contribute to the display--this reduces the aperture ratio accordingly. With a view to overcoming this defect, the inventors of the subject application proposed, in their prior U.S. patent application Ser. No. 07/733,177 filed Jul. 19, 1991, now U.S. Pat. No. 5,245,450 issued Sep. 14, 1993, a pixel structure wherein subpixel electrodes and a control capacitor electrode are formed so that they cover the gap between the subpixels and overlap the subpixels over desired areas, one of the subpixel electrodes and the control capacitor electrode are connected through a contact hole made in an insulating layer and the drive voltage is applied directly to the said one subpixel electrode to apply substantially equal voltage to the liquid crystal in the subpixel and the gap region, thereby providing for increased aperture ratio of the pixel. The pixel electrode will be described with reference to FIGS. 1 and 2.
FIG. 1 is a plan view showing one pixel region of a liquid crystal display panel of the type having pixels arranged in a matrix form, and FIG. 2 is a sectional view taken on the line II--II in FIG. 1. Reference numeral 1 indicates a transparent substrate as of transparent glass, on the interior surface of which there is formed a control capacitor electrode 2, which is, in turn, covered with an insulating layer 15 deposited almost all over the interior surface of the substrate 1. On the top of the insulating layer 15 there is formed a subpixel electrode 4.sub.2 with one corner of the pixel region cut off in a square form, and in the void area there is provided a rectangular subpixel electrode 4.sub.1 separated by a gap GP from the subpixel electrode 4.sub.2. The insulating layer 15 has a contact hole 15H made therethrough substantially at the center of the subpixel electrode 4.sub.1, the control capacitor electrode 2 and the subpixel electrode 4.sub.1 being interconnected through the contact hole 15H. Reference numeral 8 denotes a symbol indicating a thin film transistor provided near the subpixel electrode 4.sub.1. In FIG. 2 its construction is shown in section.
A gate insulating film 24 of the transistor 8 is deposited substantially all over the interior surface of the substrate 1, covering the pixel electrodes 4.sub.1 and 4.sub.2 and a semiconductor layer 8SC of the transistor 8. The transistor 8 has its source electrode 8S and gate electrode 8G connected to a source line 21 and a gate line 25 shown symbolically in FIG. 1, respectively. Reference numeral 5 represents a transparent substrate as of transparent glass, which is coated all over its interior surface with a common electrode 6. Reference numeral 12 denotes a strip-like additional capacitor electrode, which is deposited opposite a part of the subpixel electrode 4.sub.2 with the gate insulating film 24 sandwiched between them. The additional capacitor electrodes 12 of all pixels are supplied with a common potential. Between the common electrode 6 and the gate insulating film 24 there is sealed liquid crystal 7. Incidentally, the control capacitor electrode 2, the subpixel electrodes 4.sub.1, 4.sub.2 and the common electrode 6 are transparent electrodes as of ITO. The subpixel electrode 4.sub.1 has connected thereto a drain electrode 8D of the thin film transistor 8 provided in its vicinity and a voltage Va corresponding to the picture signal level is applied to the subpixel electrode 4.sub.1 via the drain electrode 8D.
The pixel of such a construction as depicted in FIGS. 1 and 2 is composed of a subpixel formed by the subpixel electrode 4.sub.1 and the gap region GP of the control capacitor electrode 2 connected thereto, and a subpixel formed by the subpixel electrode 4.sub.2. As shown in FIG. 2, liquid crystal capacitors C.sub.LC1, C.sub.LC3 and C.sub.LC2 are formed between the common electrode 6 and the subpixel electrodes 4.sub.1, 4.sub.2 and the control capacitor 2, respectively, a control capacitor electrode C.sub.C is formed between the control capacitor electrode 2 and the subpixel electrode 4.sub.2, and an additional capacitor C.sub.S is formed between the additional capacitor electrode 12 and the subpixel electrode 4.sub.2. FIG. 3 shows an equivalent circuit of the pixel depicted in FIG. 1.
The drive voltage Va supplied via the drain electrode 8D of the thin film transistor 8 is applied intact to the subpixel electrode 4.sub.1, whereas the subpixel electrode 4.sub.2 is supplied with a voltage divided by the sum of the capacitances of the liquid capacitor C.sub.LC3 and the additional capacitor C.sub.S and the capacitance of the control capacitor C.sub.C. This capacitance-divided voltage is controlled by suitably selecting the capacitance value of the control capacitor C.sub.C, i.e. the overlapping area of the subpixel electrode 4.sub.2 and the control capacitor electrode 2, and the area of the additional capacitor electrode 12. The liquid crystal panel has a threshold voltage at which the transmission of light begins (i.e. the panel is turned ON) and a voltage at which the transmission of light is saturated. By gradually increasing the drive voltage Va from a low starting voltage, the transmission of light through the regions of the subpixel 4.sub.1 and the gap GP begins (i.e. these regions are turned ON) at a certain threshold voltage, and by further increasing the voltage Va, the quantity of light that is transmitted through the regions gradually increases. By further increasing the drive voltage Va, the region of the subpixel 4.sub.2 is also turned ON and the transmission therethrough of light is finally saturated, that is, all pixel regions enter the state in which the quantity of light passing therethrough is maximum. By changing the voltage to be applied to each subpixel electrode through such control of drive voltage Va, a gray-scale display can be produced.
The liquid crystal display panel of U.S. Pat. No. 5,245,450, shown in FIGS. 1 and 2, adopts the construction in which the drive voltage Va is applied directly to the subpixel electrode 4.sub.1 via the thin film transistor 8 and the subpixel electrode 4.sub.1 is connected directly to the control capacitor electrode 2 through the contact hole 15H made in the insulating layer 15. In the case where two subpixel electrodes are used as in the above-mentioned example, if the control capacitor electrode 2 and the subpixel electrode 4.sub.2 are shorted, the subpixel electrode 4.sub.2 is also supplied with the same drive voltage Va as that applied to the subpixel electrode 4.sub.1. Hence, even if a drive voltage is applied which causes, for example, the subpixel electrode 4.sub.1 and the gap region GP to produce a gray-level display, the display is provided at the same level over the entire region of the pixel, including the subpixel electrode 4.sub.2. In this case, the quantity of light passing through the pixel is larger than in the case of no short occurring between the above-mentioned electrodes, and the pixel is perceived as being defective rather than normal. In addition, since the liquid crystal molecules are oriented obliquely over the entire region of the pixel, the visual angle dependence of the liquid crystal display panel increases, and when the display panel is applied to a color display, the pixel becomes a defective pixel which clearly differs in color and saturation than in the case where no short occurs.
The above description has been given of the light transmitting type liquid crystal display panel but the same is true of a reflecting type liquid crystal display panel wherein the subpixel electrodes 4.sub.1, 4.sub.2 and the control capacitor electrode 12 are formed by metal layers.
It is therefore an object of the present invention to provide a gray-scale liquid crystal display panel which has display pixels of a large aperture ratio.
Another object of the present invention is to provide a gray-level liquid crystal display panel which has display pixels such that the visual angle dependence does not appreciably increase even if one of the subpixel electrodes and the control capacitor electrode are shorted.